The magazine lip is generally considered to be the most mechanically vulnerable part of a gas magazine. A hefty metal object, if you drop a gas magazine and it lands top side down you can chip, crack or break your polymer magazine lip.
Breaking a magazine lip is so easy to do and yet so incredibly frustrating as the destruction of such a small cheap piece of a magazine effectively turns it into a funky smelling paperweight until you fix the problem.
If you need to replace your magazine lip then a cheap and simple lip like this is perfect. So easy and cheap to drop on, why not just buy a handful as a precautionary maintenance factor?
Even if you are the type of player who insists on using high quality lips only (big respect to you on that by the way) you can still keep a handful of these in your game bag as a stop-gap measure. Fix your mags for the second half of the game day, worry about installing your higher quality one when you get home.
If you are buying Marui Model 17 magazines, why not just throw a handful of these in your shopping basket as well to take advantage of the paid shipping for a cheap little handful of peace of mind.